In hospitals and surgeries, where a range of anaesthetic gases are used, there is a problem in dealing with the exhaled gas, such as N2O. Not all inhaled anaesthetic gas is absorbed by the patient, and it is undesirable for the clinicians in the area to inhale the gases. Although the levels are low, long term exposure may present potential health hazards. Existing prior art scavengers are designed to dispose exhaled gases without allowing them to vent to atmosphere.
In one prior art system disclosed in U.S. Pat. No. 4,527,558, the gas scavenger system includes an exhalation line that transmits exhalation gases from a patient to a surge chamber prior to eventual discharge into a vacuum system. The surge chamber provides an interface between the vacuum system and a collection manifold and is connected to the vacuum system through a predetermined sized orifice which limits the flow to the vacuum system to a maximum known flow. The surge chamber normally allows continuous flow through the orifice to the vacuum system but acts as a buffer and is sized to accumulate the volume of any exhaled flow that exceeds the continuous flow during a breathing cycle, including an excess of flow from the collection manifold under abnormal conditions such as is occasioned at a large exhalation or when a patient coughs. Therefore, the surge chamber allows time to remove the gases through the orifice and thus to prevent leakage to atmosphere. While the patient is exhaling, the exhaled gas that was not drawn into the vacuum system is drawn out from the surge chamber. As long as the surge chamber is bigger than the largest exhalation anticipated and the average exhaled flow is smaller than the constant flow, the system will prevent the exhaled gas entering the surroundings. The surge chamber itself is directly connected to atmosphere through a known, fixed resistance such that an excess in pressure in the surge chamber above that determined by the known fixed resistance is bled to atmosphere. Thus, in the event of an occlusion in the vacuum line, the patient is assured a path for exhalation. Also, in the event the vacuum system draws gas at a faster flow rate than that of waste gases entering the surge chamber from the collection manifold, gas will be drawn from atmosphere through the known, fixed resistance. The pressure is thus controlled at the collection chamber to prevent an excess of either pressure or vacuum from affecting the function of a normal demand valve or breathing apparatus supplying the anaesthetic containing gas.
In a scavenging system, all of a patient's exhalation gases should be transferred to the hospital vacuum system. The resistance to exhalation should be small, to make it comfortable for the patient, and to help achieve evacuation of exhaled gases. The patient should not be able to re-breathe exhalation gases that have already been exhaled into the scavenger. The patient should be protected from vacuum flow. The patient should still be able to exhale in the event of failure of the vacuum or it being weak. A clinician should be able to see that the scavenger is operating properly.
In the prior art system, a constant vacuum flow is drawn regardless of patient exhalation. Thus, the device may use significantly more vacuum flow than needed for the patient, wasting valuable energy and causing a drain on the hospital vacuum system. If the vacuum flow is adjusted so that it is lower, to conserve energy, some of the patient's exhaled breath may well escape to atmosphere.
The prior art systems must be turned on and off prior to and after use. That is, the vacuum connection to the device needs to be turned on and off. If a user forgets to turn the device on, then a patient may inadvertently exhale to atmosphere. If a user forgets to turn off the device after use, there will be a constant drain on the hospital vacuum system. There is also a need to adjust vacuum flow for the breathing cycle of different patients, due to, for example, size, fitness and age of a patient. In this regard, the prior art devices may have an adjustment to make the constant flow higher or lower according to patient size, necessitating yet another aspect which must be controlled by a clinician, requiring some prior judgment in order to make an adjustment.
The size of the surge chamber has to be larger than the maximum tidal volume (the largest amount that may be exhaled in one breath) of the largest patient liable to be encountered. Therefore, the surge chamber must be relatively large having a capacity in the region of 2-5 liters or around 500 mm tall and 100 mm diameter, with a weight in the region of 1 to 5 kg. This means that the scavenger is large relative to typical medical equipment such as a demand valve or a gas mixer and therefore it may occupy an undue amount of space, and be inconvenient to move. Further, the size of the device and the amount of material used in its manufacture will result in a high cost high, even though the technology may be simple.
The constant flow of air into the vacuum system can cause a background noise, where the surge chamber can act as a sounding device, amplifying the sound, which can be a strain on the user and the patient. Further, there is very limited indication that the scavenger is operating properly, as there are no moving parts to be observed.